1. Technical Field of the Invention
The present invention relates to a method of machining the shaft of a turbine rotor for a supercharger.
2. Prior Art
FIGS. 1A and 1B show the general configuration of a turbine rotor shaft with integrated turbine blades and rotor shaft. In these figures, FIG. 1A shows a completed turbine rotor shaft 1, and FIG. 1B is a view showing the turbine rotor shaft 1 separated into the turbine blade unit 2 and the rotor shaft 3. The right hand end of the turbine rotor shaft 1 in FIG. 1A, is attached to the compressor (not illustrated) with screws to form the supercharger assembly. Such turbine rotor shafts 1, particularly small types, rotate at speeds as high as several tens or several hundreds of thousands of revolutions per minute. Therefore it is very important that they should be accurately balanced. Consequently, imbalance of the turbine rotor shaft 1 is measured by a dynamic balancing test, and then parts A and B (2 locations), hatched in the figures, are ground to eliminate the imbalance.
FIG. 2 is a flow chart of the processes used to machine a turbine rotor shaft according to a conventional method known in the prior art, and FIGS. 3A to 3D are typical views showing the corresponding steps. As shown in FIGS. 2 and 3, first the joint portion of a precision cast turbine blade unit 2, is machined, and the rotor shaft 3 is machined to an approximate shape leaving a finishing allowance (FIGS. 3A, 3B). Next, the joint portion of the turbine blade unit 2 and the rotor shaft 3 are joined by electron beam welding into an integrated turbine rotor shaft 1 (FIG. 3C). Then, the rotor shaft is finish machined, hardened (by a nitriding process or by high-frequency quenching), and the shaft and the outer periphery of the turbine blades are ground (3D). Finally, the degree of imbalance is measured by a dynamic balancing test, part of the turbine blade unit are cut to correct the imbalance, and the turbine rotor shaft 1 is completed.
FIGS. 4A and 4B show a process for machining the joint portion of the precision cast turbine blade unit 2, before and after machining, respectively. As shown in the figures, the joint portion of the precision casting is bored beforehand with a boss hole 2a, and in this machining process, the end surface 2b and the inner surface 2c of the joint portion are machined using the end surface A of the joint end and the outer periphery B of the turbine blade unit as the reference surfaces. In addition, the center hole 2d of the turbine blade unit cannot be centered when the turbine blade unit is unattached therefore the rotor shaft 3 is first welded and finish machined, and then the center hole is machined.
However, there is a problem that a large amount of imbalance is produced in the turbine blade unit 2, when this machining process according to a conventional method known in the prior art is used.
FIG. 5 is a view illustrating a process for welding the turbine blade unit 2 and the rotor shaft 3 by electron beam welding. As shown in the figure, according to a conventional electron beam welding method, the end surface 3a of the rotor shaft 3 is inserted into the inner surface 2c of the turbine blade unit 2, the entire body is held vertically using a welding jig 4, and the turbine blade unit 2 is pressed in by a ball 5. Next, in this state, the joint portion is tack welded by the head 6 of the electron beam device (with a welding angle of, for example, 360xc2x0), and finally welded (with a welding angle of, for instance, 830xc2x0).
However, this welding process according to a conventional method in the prior art is accompanied by the problem that the turbine blade unit 2 and the rotor shaft 3 are welded at a slightly skewed angle to each other.
Therefore, in the aforementioned balancing adjustment at the final stage, the amount of imbalance is often excessively large, resulting in a long time needed to make repairs, rejection of inferior workpieces, etc.
The present invention is aimed at solving these problems. That is, an object of the present invention is to provide a method of machining a turbine rotor shaft for superchargers, wherein the degree of imbalance that occurs unavoidably with conventional machining methods can be greatly reduced, thus the time needed to correct the imbalance and the yield of the workpieces can be increased.
Conventionally, the joint portion is machined using the outer periphery of the turbine blade unit as the reference for machining. However, originally the turbine blade unit was precision cast, and the blade portions, used as machining references, have complicated shapes with thin walls, and because the cast portions cool quickly, they are subject to large deformations caused by shrinkage stresses. Hence, the dimensional accuracy of these portions is not as high as is considered necessary for use as a machining reference (about xc2x10.02 mm), that is, actually the accuracy is about 0.2 mm. As a result, the center of the joint portion machined using the outer periphery of blades as the machining reference deviates from the center of balance of the entire turbine blade unit, so that the deviation thereof causes an imbalance of the turbine rotor shaft as a whole, as revealed in the results of measurements to be described later.
On the other hand, the center of balance of the turbine blade unit is in the center portion which cools slowly, as the ratio of the mass to the surface area thereof is larger than that of the blades. In other words, this portion is less affected by shrinkage stresses, and the accuracy thereof can be maintained rather easily. As a consequence, the finished accuracy of a boss hole in the center portion of a precision casting is as high as about xc2x10.01 mm, as shown by the results of measurements.
The first embodiment of the present invention is established based on the above-mentioned novel knowledge. More explicitly, according to the present invention, a cylindrical boss hole (2a) with a predetermined necessary tolerance is constructed in the joint portion of the turbine blade unit (2) which joins to the rotor shaft (3), one end of the rotor shaft previously finish machined is inserted into the boss hole, and the joint portion is welded by electron beam welding, as a novel method of machining the turbine rotor shaft of a supercharger.
Using this configuration, the imbalance that was unavoidably produced when cutting the joint portion according to the conventional method can be eliminated, and the rotor shaft can be welded with the center line of the boss hole (2a) near to the center of balance of the precision casting.
Next, in a conventional electron beam welding process known in the prior art, the turbine rotor shaft is subject to deflection due to shrinkage stresses caused when the molten metal solidifies after welding. As a result, conventional turbine rotor shafts are deflected by a mean angle of 0.14xc2x0 and a 3"sgr" value of 0.34xc2x0 according to the results of measurements. This angle of deflection corresponds to a mean runout of 0.45 mm and a 3"sgr" value of 1.09 mm at the tip of the shaft, even for the small turbine rotor shafts used for passenger cars. If such a deflection must be eliminated by grinding the outer periphery of the turbine blades, one skilled in the art may easily understand that it results in a very small yield.
Another idea that might be proposed is to mechanically clamp the turbine blades and the rotor shaft to reduce such a deflection as described above while joining them, however, this idea cannot be applied so widely and is not desirable in terms of production efficiency because the number of factors that must be controlled, such as clamping pressure, verticality of the end surface and accuracies of the jigs increases, and also a large variety of jigs are required depending on the total length of the shaft.
The second embodiment of the present invention takes into account the novel knowledge described above. In practice, according to the present invention, a plurality of components are welded on the same axis; while the plurality of components are held in position on the same axis, the joint portions are simultaneously welded together by electron beam welding at a plurality of spots spaced at equal angles around the circumference, which is a method of producing the turbine rotor shaft for a supercharger according to the present invention. According to the preferred embodiment of the present invention, the aforementioned plurality of components are the turbine blade unit (2) and the rotor shaft (3), and one end (3a) of the rotor shaft is inserted into a boss hole (2a) formed in the joint portion of the turbine blade unit, and while both the turbine blade unit and the rotor shaft are held in position on the same axis, the joint portions are simultaneously welded by electron beams at a plurality of spots spaced at equal angles around the circumference.
In this configuration, the joint portions are welded simultaneously at a number of locations spaced at equal angles, thus the effects of shrinkage as the molten metal solidifies are balanced as they are spaced at the same angle and bending distortions are reduced. The time interval and power input at each spot to be irradiated can be easily controlled by adjusting the equipment, and moreover, there are no additional factors to be controlled, so that the method is effective for increasing productivity. In addition, the quality of a workpiece is not affected by external factors such as the accuracy of jigs, therefore by applying the method, a high quality product can be manufactured.
According to a conventional electron beam welding process known in the prior art, the turbine blade unit (2) and the rotor shaft (3) are welded together, and then the rear surface of the turbine blade unit is pressed against the surface plate of a machining jig, and the outer periphery of the turbine blades is clamped by a collet chuck, and the outer periphery is machined.
However, according to this method, another imbalance is produced because the center line of the machining jig itself deviates from that of the main shaft due to the effect of the collet chuck and, as described before, the center of the circle formed by the outer periphery of the turbine blades is offset from the center of balance. In other words, although several samples are used to adjust the jig, the variations between the products are large and they cannot be accurately positioned. Also, bending distortion caused by the aforementioned electron beam welding causes a deflection of the workpiece before machining, consequently the cutting process is intermittent in practice, therefore, the machining stresses produced in the workpiece are uneven, resulting in a runout after machining is remained.
This problem is affected by variations between each product, the skill of the operators, etc., and cannot be improved so easily. In addition, there are many unstable factors concerning the accuracy of the reference sample used for machining, as well as in the machining of the joint parts, and consequently the machining operation is also another cause of imbalance.
The third embodiment of the present invention takes the above-mentioned novel knowledge into account. According to this embodiment, a method of machining a turbine rotor shaft for a supercharger is proposed, wherein the rotor shaft (3) is machined to a finished state separately, then one end of the rotor shaft is inserted into the boss hole (2a) in the turbine blade unit (2) and welded, next, using the outer periphery and the end surface of the rotor shaft as machining references, the turbine blades are machined.
Thus configured, because the rotor shaft (3) has been finish machined separately the accuracy of machining the rotor shaft can be improved, and the imbalance can be minimized. Afterwards, since one end of the rotor shaft is inserted in the boss hole (2a) in the turbine blade unit (2), and then the turbine blades are machined using the outer periphery and the end surface of the rotor shaft as machining references, the imbalance of the turbine blades can also be kept to a minimum.